1. Field of the Invention
The present invention relates to an image reading device including a line image sensor and a control method thereof.
2. Description of the Related Art
The existing so-called sheet-scan type (also known as sheet-through type) document readers feed a document original in the sub-scanning direction over an exposure glass. In addition they are equipped with a line image sensor that reads one line of an image in the main scanning direction at a reading position on the exposure glass. If dirt happens to be adhering to the exposure glass, the document reader reads the dirt as well, and the resulting image includes a vertical streak, which is the image of the dirt.
When such vertical steaks appear, the user has no option but to clean the exposure glass and repeat the reading operation.
Japanese Patent Application Laid-open No. 2002-251605, for example, discloses a technology to address the problem.
In this technology, to detect dirt on the reading surface, a conveyor belt is read without placing the document original on the exposure glass. Whether dirt is on the conveyor belt or on the reading surface is determined by whether or not the image of the streak is continuous in the sub-scanning direction.
If it is determined that dirt is present on the reading surface, the carriage is moved by a specific amount until no dirt is detected. However, if no position is dirt-free, it is determined that sheet-through reading is not possible, and a message indicating that there is presence of dirt is output.
However, the technology described above fails if dirt adheres to the reading surface when reading is underway. Further, the vertical streak on the conveyor belt is practically indistinguishable from the vertical streak caused by the dirt on the reading surface.